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245
body size Clarke, 1993. One-way ANOSIM and two-way crossed ANOSIM were used to compare the dietary data for the corresponding size classes of intermoult crabs in the
Peel–Harvey and Leschenault estuaries, in order to determine whether the dietary compositions of intermoult crabs in these two estuaries were significantly different. N.B.
Although Clarke and Warwick 1994 recommend using replicates for tests employing ANOSIM, the contents of each crab stomach replicate often comprise only three or
four of the 14 dietary categories recorded overall in stomachs. Thus, to reduce the number of zero values and thereby construct a more appropriate similarity matrix for
testing with ANOSIM, the dietary data for up to six groups of three randomly-selected stomachs in each size class of each shell state in each estuary were pooled. It should be
noted that, when different trios of randomly-selected stomachs were subjected to ANOSIM, they always yielded the same results. In those cases where significant
differences were detected with ANOSIM, similarity percentages SIMPER were employed to identify those dietary categories which made the greatest contribution to
those differences Clarke, 1993. Multivariate dispersion MVDISP was used to determine the degree of dispersion of the dietary samples of crabs representing different
size classes and shell states and occupying different estuaries Somerfield and Clarke, 1997.
3. Results
3.1. Stomach fullness and overall dietary compositions in the Peel–Harvey Estuary Since ANOVA showed that the values for the stomach fullness of females and males
in neither the premoult, nor recently moulted, nor intermoult shell states differed significantly, the data on fullness for the two sexes in each of these shell states have
been pooled. Food was found in the stomachs of 396 of the 455 Portunus pelagicus collected from the Peel–Harvey Estuary and, in the case of stomachs that contained
food, the mean fullness
61 S.E. was 4.760.1. However, ANOVA demonstrated that ´
stomach fullness varied significantly among shell states F 5 4.11, P , 0.05. Scheffe’s
¯ a posteriori test showed that the stomach fullness of recently-moulted crabs x
5 ¯
5.9 60.3 was significantly greater than that of intermoult crabs x54.560.1, which in
¯ turn was significantly greater than that of premoult crabs x
52.060.5. ANOSIM demonstrated that the dietary compositions of neither recently-moulted nor
intermoult crabs in the Peel–Harvey Estuary, nor of intermoult crabs in the Leschenault Estuary, differed significantly P
.0.05 between the two sexes. For this reason, the dietary data for the females and males of each size class of both recently-moulted crabs
and intermoult crabs in the Peel–Harvey Estuary and of intermoult crabs in the Leschenault Estuary were pooled.
Calcareous material was found in the stomachs of over 50 of premoult and over 70 of recently-moulted crabs in the Peel–Harvey Estuary and contributed over 50 to
the volume of the diets of the crabs representing those two shell states Table 1. This material, which comprised fragments of the shells of bivalve and gastropod molluscs,
was more weathered and broken up in the stomach contents of recently-moulted than
246 S
. de Lestang et al. J. Exp. Mar. Biol. Ecol. 246 2000 241 –257 Table 1
Frequency of occurrence F and volumetric contributions V of different dietary categories to the stomach contents of premoult, recently moulted and intermoult Portunus pelagicus in the Peel–Harvey Estuary
and of intermoult P . pelagicus in the Leschenault Estuary
Estuary Peel–Harvey Estuary
Leschenault Estuary
Shell state Premoult
Recently moulted Intermoult
Intermoult Number of stomachs
15 35
320 95
Dietary categories F
V F
V F
V F
V Small bivalves
13.3 15.5
58.5 22.3
47.2 20.9
24.2 9.2
Large bivalves –
– 1.5
,0.1 10.6
4.0 5.3
0.7 Small gastropods
– –
15.4 2.6
10.0 3.3
22.1 5.8
Cephalopods –
– –
– 0.3
0.2 1.1
0.6 Gammarid amphipods
– –
10.8 3.4
30.3 19.8
52.6 33.0
Small decapods –
– –
– 5.6
4.2 3.2
0.5 Decapod shell fragments
– –
21.5 9.4
2.5 1.3
9.5 2.2
Tanaids 6.7
1.0 –
– 9.1
6.8 6.3
3.4 Polychaetes
26.7 20.6
21.5 3.8
47.8 25.1
36.8 21.6
Echinoderms –
– 1.5
0.4 6.3
0.5 1.1
0.8 Teleosts
6.7 2.1
1.5 0.7
14.1 5.9
11.6 6.0
Calcareous material 53.3
58.8 73.8
54.0 13.4
3.1 26.3
9.9 Plant material
6.7 2.1
26.2 3.2
15.9 5.0
17.9 6.4
premoult crabs. Polychaetes, especially the nereid Ceratonereis aequisetis Augener, and small bivalve molluscs, particularly the galeommatid Arthritica semen Menke,
were often consumed by crabs of these two shell states. However, the frequency with which polychaetes occurred in stomachs and their volumetric contributions were greater
in premoult than recently-moulted crabs, while the reverse was true for small bivalves. Shell fragments of large decapods, especially Portunus pelagicus and Ovalipes au-
straliensis Stephenson and Rees, gammarid amphipods and small gastropods, mainly Tatea spp., which were each consumed by more than 10 of recently-moulted crabs and
contributed 9.4, 3.4 and 2.6 to the dietary volume of these crabs, respectively, were never found in the stomachs of premoult crabs Table 1.
Small bivalves, represented almost exclusively by A . semen, and polychaetes,
especially C . aequisetis, were the dietary categories most frequently consumed by far by
intermoult P . pelagicus in the Peel–Harvey Estuary, each being found in ca. 47 of
stomachs and contributing 20.9 and 25.1 to the total dietary volume, respectively Table 1. Gammarid amphipods were consumed by nearly one third of intermoult crabs
and comprised nearly 20 of their dietary volume. No other category contributed more than 7 to the total dietary volume Table 1.
3.2. Ontogenetic changes in diet in the Peel–Harvey Estuary In recently-moulted crabs in the Peel–Harvey Estuary, calcareous material contributed
between 47 and 55 to the total dietary volume of each of the four size classes in which the carapace widths lay between 30 and 159 mm Fig. 1. Although crabs
,90 mm CW
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247
Fig. 1. Percentage volumetric composition of different dietary categories in sequential 30 mm carapace width classes of recently-moulted and intermoult Portunus pelagicus in the Peel–Harvey Estuary and of intermoult P
. pelagicus in the Leschenault Estuary. Numbers above columns denote the number of crabs that contained
identifiable dietary categories in their stomachs.
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. de Lestang et al. J. Exp. Mar. Biol. Ecol. 246 2000 241 –257
also consumed considerable amounts of small bivalves, i.e. ca. 30, this category made only a small contribution to the diets of crabs
.120 mm CW. In contrast, the contributions made by both polychaetes and the shell fragments of large decapods
increased progressively as crabs of this shell state increased in size Fig. 1. The diets of small intermoult crabs in the Peel–Harvey Estuary, i.e.
,30 mm, contained large amounts of crustaceans, such as amphipods and tanaids, particularly
Tanais dulongii Thomson, and also small bivalve molluscs, which collectively con- tributed just over 90 to the total volume of the diet Fig. 1. As P
. pelagicus increased in size, the contribution of amphipods and, to a lesser extent, that of tanaids, declined
progressively, while particularly those of polychaetes and teleosts Favonigobius lateralis Macleay and also small decapods Penaeus latisulcatus Kishinouye and
calcareous and plant material increased.
Classification, using dietary data for the different size classes of recently-moulted and intermoult crabs in the Peel–Harvey Estuary, separated the dietary samples into two
large groups, i.e. A and B Fig. 2a. Group A contained all of the samples from recently-moulted crabs, while group B comprised all of those from intermoult crabs.
Within groups A and B, the dietary samples from the two smallest size classes, i.e. groups D and F, respectively, were separated from those of the larger size classes of that
shell state, i.e. groups C and E, respectively Fig. 2a.
The results of ordination paralleled those of classification, with all of the points corresponding to the samples of recently-moulted crabs forming a discrete group, lying
on the left of the plot and well separated from the discrete group of intermoult crabs, which lay on the right side of the plot Fig. 2a and b. The samples for intermoult crabs
followed a downwards progression on the plot according to the size of the crabs, with those representing the smaller crabs at the top and those of the largest crabs at the
bottom Fig. 2b. ANOSIM showed that the dietary compositions of intermoult crabs differed significantly with body size P
,0.001. SIMPER showed that the dietary differences in intermoult crabs were mainly attributable to the consumption of greater
volumes of amphipods and tanaids by small crabs and of far larger amounts of polychaetes and teleosts by larger crabs. Although the trend exhibited on the ordination
plot by the samples for recently-moulted crabs was less pronounced than with intermoult crabs, the samples for the two largest size classes of recently-moulted crabs still likewise
lay below those for the two smallest size categories Fig. 2b. However, ANOSIM failed to detect a significant difference in the dietary compositions of the different size classes
of recently-moulted crabs, which is presumably due to the similar and consistently high contributions made by calcareous material to the diets of each size class. The latter view
is consistent with the fact that, when this dietary category was excluded from the ANOSIM test, the dietary compositions of the different size classes became significantly
different P
,0.01. The more pronounced variation in the dietary compositions of the different size groups of intermoult crabs than recently-moulted crabs is reflected in the
greater multivariate dispersion values, i.e. 1.02 vs. 0.55. 3.3. Comparisons between diets in the Peel–Harvey and Leschenault estuaries
The three dietary categories most frequently preyed upon by intermoult P . pelagicus
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249
Fig. 2. Classification a and two dimensional multi-dimensional scaling MDS ordination b of the mean percentage volumetric contributions of the different dietary categories to the diets of different size classes of
recently-moulted and intermoult Portunus pelagicus in the Peel–Harvey Estuary.
in the Peel–Harvey Estuary, i.e. small bivalves, gammarid amphipods and polychaetes, were also those that were most frequently ingested by intermoult crabs in the
Leschenault Estuary Table 1. Furthermore, gammarid amphipods and polychaetes both contributed between ca. 20 and 33 to the dietary volume of intermoult crabs in both
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. de Lestang et al. J. Exp. Mar. Biol. Ecol. 246 2000 241 –257
estuaries. The volume of calcareous material made greater contributions to the diets of intermoult crabs in the Leschenault than Peel–Harvey estuaries, i.e. 9.9 vs. 3.1. Each
of the other nine dietary categories ingested by intermoult crabs in the Peel–Harvey Estuary were also consumed by intermoult crabs in the Leschenault Estuary Table 1.
The diet of intermoult P . pelagicus underwent the same types of ontogenetic changes
in the Leschenault Estuary as in the Peel–Harvey Estuary Fig. 1. Thus, for example, the contributions made by polychaetes increased markedly with body size, whereas the
reverse occurred with amphipods. However, calcareous material tended to be consumed in slightly greater volumes by the larger size classes of crabs in the Leschenault than
Peel–Harvey estuaries, whereas the reverse applied to small decapods Fig. 1.
Classification, using dietary data for the different size classes of intermoult crabs in the Peel–Harvey and Leschenault estuaries, separated the samples of the smallest size
class of crabs in the Leschenault Estuary from those of all other dietary samples Fig. 3a. Within the latter large group, i.e. A, the samples were separated into two main
groups B and C, with the former containing only samples from large crabs CW 90
mm. In group C, which contained all but one of the samples of the smaller crabs CW
,90 mm, the samples were separated according to the body size of the crab rather than the estuary in which the crab had been caught Fig. 3a.
Ordination of the mean percentage volumetric contributions of the dietary categories for the five successive size classes of intermoult crabs in each of the two estuaries
resulted in the points for samples representing the smallest size classes from each estuary lying towards the top of the plot Fig. 3b. In contrast, those for the next two size
classes, i.e. 30–59 and 60–89 mm CW, lay in the middle of the plot and those of the two larger size classes lay towards the bottom. There was no consistent trend for the samples
of corresponding size classes from one estuary to lie either above or below or to the left or right of those from the other estuary Fig. 3b. The dispersion values recorded for the
dietary samples of crabs from the Peel–Harvey and Leschenault estuaries were very similar, i.e. 1.02 vs. 1.00. ANOSIM showed that, as with crabs from the Peel–Harvey
Estuary, there was a significant size-related difference in intermoult crabs in the Leschenault Estuary P
,0.05. SIMPER likewise demonstrated that the size-related differences in dietary compositions in the Leschenault Estuary were attributable to
differences in the greater contributions made by amphipods and tanaids to the diets of smaller crabs and by polychaetes and teleosts to those of larger crabs. A two-way
crossed ANOSIM, which took the size of crabs into account, showed that the dietary compositions of intermoult crabs in the two estuaries were not significantly different
P
.0.05 and a series of one-way ANOSIMs showed that the dietary compositions of each size class in the Peel–Harvey Estuary did not differ significantly from that of its
corresponding size class in the Leschenault Estuary P .0.05.
4. Discussion
